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A star system where gas and dust have formed into a disk around a newly formed star. The leftover disk will most likely form planets, comets and asteroids. Credit: NASA

No one is ever excited when the topic of “dust” is brought up. Usually dust is a hindrance – something you sweep away during spring-cleaning, or an annoyance because your allergies can’t handle it. But for astronomers, finding dust around another star – i.e., circumstellar dust – is like finding the next piece of an interstellar puzzle. That’s because circumstellar dust holds clues to understanding not only the origins of planets outside of our solar system, but also gives us a leg up in figuring out our place in the Universe.

Before we can uncover the secret of how dust and exoplanets are linked, we need to understand what happens after a planetary system forms. Once a star is created, it leaves behind a large disk of gas and dust. The gas and dust start sticking together and coalescing into larger objects such as planets, asteroids and comets. The central star removes the remaining gas and dust either by accreting it, or by throwing it out of the system with its stellar wind and radiation pressure. About 10 million years or so after the star forms, all of the leftover dust and gas have either been ejected out of the system, eaten up by the star, or used to form planets, asteroids and comets. But here’s a mystery: astronomers have found stars much older than 10 million years with circumstellar dust.

So then where does all this dust come from if we know it shouldn’t be there?

One word – collisions.

Large planets will gravitationally tug on smaller bodies such as asteroids and comets as they pass by. And if these asteroids and comets are pulled hard enough, they will eventually collide with one another! This can be catastrophic and destructive, breaking the original asteroid into smaller chunks. These chunks can then collide and become still smaller and smaller and smaller – grinding down the original body into dust. The dust then forms a disk around the star — what astronomers call a “debris disk.”

Had there not been large planets around these stars, there might not have been any destructive collisions, which in turn would not produce any dust for us to find. In other words, finding dust around a star is like seeing a large signpost saying “PLANETS! Come and get ‘em’!” Of course, initially we don’t actually see the planets. But knowing a debris disk exists around these stars tells us there is a good chance we’ll find them.

Detecting Dust

Emission spectrum seen from a star that does and does not have dust around it. A star with dust will have excess infrared radiation compared to the emission from the star. Credit: NASA

Now, astronomers typically look for debris disks by measuring the infrared light coming from a star. Dust around a star will warm as it soaks up the light from the central star. As it heats up, it will start emitting its own light in the infrared – just like a stove-top coil will begin to feel hot before you see it glow red. The amount of infrared light the dust gives off, combined with what the star produces, will be more than the amount of infrared light produced only by the star. This excess light is what betrays the presence of dust in the system. This dust is usually 10 to 100 microns in size, or roughly the thickness of a human hair.

Astronomers have found infrared excess emission – most likely caused by debris disks – in over 1,000 star systems, all of which have the potential to host planetary systems.

But what about planets? How can we confirm that planets are responsible for the creation of the dust we’re seeing?

Left: Hubble image of the Formalhaut dust ring. Credit: Kalas et al. 2012. Center: Beta pic debris disk by Smith and Terrile. Right: Beta pic b planet over plotted on image of the beta pic debris disk. The stars in each image are blocked out by a coronagraph.

In some cases, taking an image of a system that has a known debris disk reveals much more than one can discern from just measuring the amount of infrared light produced by a star. In 2005, GPIES’ own Dr. Paul Kalas and his team used the Hubble Space Telescope to image the debris belt of Fomalhaut – a star roughly 16 times brighter and almost four billion years younger than our sun. These images show a sharp, eccentrically misaligned ring, which hints that a large planet might be orbiting inside the ring. Follow-up observations inspired by this suspicion revealed a planet, Fomalhaut b, though not the one thought responsible for the shape of the debris disk.

Hubble Space Telescope observation of the debris ring around Fomalhaut. The inner edge of the disk may have been shaped by the orbit of Fomalhaut b, at lower right.

Another star system whose imaged disk betrays the existence of a planet is “beta Pictoris” – or beta Pic for short. Beta Pic’s debris disc was the first to be imaged. In 1984, Dr. Brad Smith and Dr. Rich Terrile took an image of the beta Pic disk by blocking out the star’s light using a coronagraph, which revealed an edge-on disk. The disk of this 20-million- year-old star – much younger than our sun – is full of warps and substructures. Such structures in the disk led astronomers to believe that a large planet may be influencing its shape.

And then..

In 2008, astronomers finally discovered the faint signal of a planet eight times the mass of Jupiter, captured in images taken by the Very Large Telescope in 2003. These two systems are only the tip of the iceberg, as many more such stars have been studied, all of which show a wide variety of disk structures and hint at the presence of planets hidden behind the dust.

How does this all link back to us here on Earth?

Although the puzzle of each exosolar system becomes clearer when we study its debris disk, information gleaned from every system can be used to deduce whether a planetary system like ours can form elsewhere in the Universe. This is mainly because the interaction of our solar system’s planets and debris disk – which is made up of the asteroid belt between Mars and Jupiter, and the Kuiper belt way past Neptune – have heavily influenced the current structure of our solar system.

Roughly 3.7 billion years ago, Jupiter and Saturn began a gravitational dance in which the orbital periods of the titans lined up, and for every orbit around the Sun Jupiter made, Saturn made two. This enhanced gravitational interaction forced Saturn to migrate slowly away from the Sun, pushing Uranus and Neptune further out in the solar system as well. Neptune fatefully crashed into the outer Kuiper belt, sending large ice and rock bodies hurling all over the solar system, and turning its inner region– and Earth – into an interplanetary shooting range. A large amount of dust would have been created during this time period, one quite noticeable to any alien observing our system.

Although this event made the primordial Earth a hellish place to live, there is some good news. Some scientists have theorized that a large portion of Earth’s oceans were fed by the transport of these water-rich comets and asteroids from the Kuiper belt during the “Late Heavy Bombardment.”. And so had it not been for the relationship between our planets and debris belts, Earth would probably not exist as we know it.

We have seen evidence of similar catastrophic events around the young “eta Corvi” system. Using spectroscopic data obtained in 2010 from the Spitzer Space Telescope, Dr. Carey Lisse’s team discovered that for some reason, large numbers of cometary bodies from the outer regions of this system were colliding with a planetary sized body in the inner regions, and releasing water ice dust whose total mass was about 0.1% of all the water in Earth’s oceans!

Spitzer

The similarity between eta Corvi and our early solar system is uncanny. Additionally, it’s exciting to think that the events that may have allowed for life to arise on Earth are currently going on around other stars – events that require a symbiotic relationship between planets and the remnant asteroid and comet population.

Stunning exoplanet images and spectra from the first year of science operations with the Gemini Planet Imager (GPI) were featured today in a press conference at the 225th meeting of the American Astronomical Society (AAS) in Seattle, Washington. The Gemini Planet Imager GPI is an advanced instrument designed to observe the environments close to bright stars to detect and study Jupiter-like exoplanets (planets around other stars) and see protostellar material (disk, rings) that might be lurking next to the star.

Figure 1. GPI imaging of the planetary system HR 8799 in K band, showing 3 of the 4 planets. (Planet b is outside the field of view shown here, off to the left.) These data were obtained on November 17, 2013 during the first week of operation of GPI and in relatively challenging weather conditions, but with GPI’s advanced adaptive optics system and coronagraph the planets can still be clearly seen and their spectra measured (see Figure 2). Image credit: Christian Marois (NRC Canada), Patrick Ingraham (Stanford University) and the GPI Team.

Figure 2. GPI spectroscopy of planets c and d in the HR 8799 system. While earlier work showed that the planets have similar overall brightness and colors, these newly-measured spectra show surprisingly large differences. The spectrum of planet d increases smoothly from 1.9-2.2 microns while planet c’s spectrum shows a sharper kink upwards just beyond 2 microns. These new GPI results indicate that these similar-mass and equal-age planets nonetheless have significant differences in atmospheric properties, for in-stance more open spaces between patchy cloud cover on planet c versus uniform cloud cover on planet d, or perhaps differences in atmospheric chemistry. These data are helping refine and improve a new generation of atmospheric models to explain these effects. Image credit: Patrick Ingraham (Stanford University), Mark Marley (NASA Ames), Didier Saumon (Los Alamos National Laboratory) and the GPI Team.

Marshall Perrin (Space Telescope Science Institute), one of the instrument’s team leaders, presented a pair of recent and promising results at the press conference. He revealed some of the most detailed images and spectra ever of the multiple planet system HR 8799. His presentation also included never-seen details in the dusty ring of the young star HR 4796A. “GPI’s advanced imaging capabilities have delivered exquisite images and data,” said Perrin. “These improved views are helping us piece together what’s going on around these stars, yet also posing many new questions.”

The GPI spectra obtained for two of the planetary members of the HR 8799 system presents a challenge for astronomers. GPI team member Patrick Ingraham (Stanford University), lead the paper on HR 8799. Ingraham reports that the shape of the spectra for the two planets differ more profoundly than expected based on their similar colors, indicating significant differences between the companions. “Current atmospheric models of exoplanets cannot fully explain the subtle differences in color that GPI has revealed. We infer that it may be differences in the coverage of the clouds or their composition.” Ingraham adds, “The fact that GPI was able to extract new knowledge from these planets on the first commissioning run in such a short amount of time, and in conditions that it was not even designed to work, is a real testament to how revolutionary GPI will be to the field of exoplanets.”

Perrin, who is working to understand the dusty ring around the young star HR 4796A, said that the new GPI data present an unprecedented level of detail in studies of the ring’s polarized light. “GPI not only sees the disk more clearly than previous instruments, it can also measure how polarized its light appears, which has proven crucial in under-standing its physical properties.” Specifically, the GPI measurements of the ring show it must be partially opaque, implying it is far denser and more tightly compressed than similar dust found in the outskirts of our own Solar System, which is more diffuse. The ring circling HR 4796A is about twice the diameter of the planetary orbits in our Solar System and its star about twice our Sun’s mass. “These data taken during GPI commissioning show how exquisitely well its polarization mode works for studying disks. Such observations are critical in advancing our understanding of all types and sizes of planetary systems – and ultimately how unique our own solar system might be,” said Perrin.

These GPI observations reveal a complex pattern of variations in brightness and polarization around the HR 4796A disk. The western side (tilted closer to the Earth) appears brighter in polarized light, while in total intensity the eastern side appears slightly brighter, particularly just to the east of the widest apparent separation points of the disk. Reconciling this complex and apparently-contradictory pattern of brighter and darker regions required a major overhaul of our understanding of this circumstellar disk. Image credit: Marshall Perrin (Space Telescope Science Institute), Gaspard Duchene (UC Berkeley), Max Millar-Blanchaer (University of Toronto), and the GPI Team.

Figure 4. Diagram depicting the GPI team’s revised model for the orientation and composition of the HR 4796A ring. To explain the observed polarization levels, the disk must consist of relatively large (> 5 µm) silicate dust particles, which scatter light most strongly and polarize it more for forward scattering. To explain the relative faintness of the east side in total intensity, the disk must be dense enough to be slightly opaque, comparable to Saturn’s optically thick rings, such that on the near side of the disk our view of its brightly illuminated inner portion is partially obscured. This revised model requires the disk to be much narrower and flatter than expected, and poses a new challenge for theories of disk dynamics to explain. GPI’s high contrast imaging and polarimetry capabilities together were essential for this new synthesis. Image credit: Marshall Perrin (Space Telescope Science Institute).

During the commissioning phase, the GPI team observed a variety of targets, ranging from asteroids in our solar system, to an old star near its death. Other teams of scientists have been using GPI as well and already astronomers around the world have published eight papers in peer-reviewed journals using GPI data. “This might be the most productive new instrument Gemini has ever had,” said Professor James Graham of the University of California, who leads the GPI science team and who will describe the GPI exoplanet survey in a talk scheduled at the AAS meeting on Thursday, January 8th.

The Gemini Observatory staff integrated the complex instrument into the telescope’s software and helped to characterize GPI’s performance. “Even though it’s so complicated, GPI now operates almost automatically,” said Gemini’s instrument scientist for GPI Fredrik Rantakyro. “This allows us to start routine science operations.” The instrument is now available to astronomers and their proposals are scheduled to start ob-serving in early 2015. In addition, “shared risk” observations are already underway, starting in November 2014.

The one thing GPI hasn’t done yet is discovered a new planet. “For the early tests, we concentrated on known planets or disks” said GPI PI Bruce Macintosh. Now that GPI is fully operational, the search for new planets has begun. In addition to observations by astronomers world-wide, the Gemini Planet Imager Exoplanet Survey (GPIES) will look at 600 carefully selected stars over the next few years. GPI ‘sees’ planets through the infrared light they emit when they’re young, so the GPIES team has assembled a list of the youngest and closest stars. So far the team has observed 50 stars, and analysis of the data is ongoing. Discovering a planet requires confirmation observations to distinguish a true planet orbiting the target star from a distant star that happens to sneak into GPI’s field of view – a process that could take years with previous instruments. The GPIES team found one such object in their first survey run, but GPI observations were sensitive enough to almost immediately rule it out. Macintosh said, “With GPI, we can tell almost instantly that something isn’t a planet – rather than months of uncertainty, we can get over our disappointment almost immediately. Now it’s time to find some real planets!”

The Gemini Observatory consists of twin 8.1-meter diameter optical/infrared telescopes located on two of the best observing sites on the planet. From their locations on mountains in Hawai‘i and Chile, Gemini Observatory’s telescopes can collectively access the entire sky.
Gemini was built and is operated by a partnership of six countries including the United States, Canada, Chile, Australia, Brazil and Argentina. Any astronomer in these countries can apply for time on Gemini, which is allocated in proportion to each partner’s financial stake.

Yesterday, we had a chance to see the telescope in all of its glory. And it is HUGE!

The Gemini South Telescope with the dome lights on.

It really makes you appreciate the amount of equipment you need to directly image these faint extrasolar planets that are orbiting other stars. Andrew, the telescope operator, then pointed the telescope down so that we could get some nice photographs with the 8-meter mirror. Here’s my telescope selfie:

Telescope selfie!

The 8 meter mirror is so big it’s hard to fit into one single shot. This was the best I could do. Although some others are a bit more serious about their photography…

Markus sprawling out to get a nice shot of Lee, a journalist visiting us, with the telescope.

Before the sun fully set, I ran outside to grab this image of the telescope dome open.

The Gemini Observatory consists of twin 8.1-meter diameter optical/infrared telescopes located on two of the best observing sites on the planet. From their locations on mountains in Hawai‘i and Chile, Gemini Observatory’s telescopes can collectively access the entire sky.
Gemini was built and is operated by a partnership of six countries including the United States, Canada, Chile, Australia, Brazil and Argentina. Any astronomer in these countries can apply for time on Gemini, which is allocated in proportion to each partner’s financial stake.